2. 10.2 Galaxies Galaxies are a collection of stars, gas and dust held together by gravity Our star, the sun, is one of 100 million stars in the Milky Way galaxy - and there are 125 billion galaxies in the universe! A galaxy is a large group of stars. A nebula is a cloud of gas and dust in space that often produces a new star, or is the remains of an old star. See pages 356 - 360

3. Galaxies are classified by shape Spiral galaxy – looks like a pinwheel with the bulge in the centre containing the oldest stars elliptical galaxy – shaped like a sphere or an ellipse; contains some of the oldest stars irregular galaxy – no set shape; mix of new stars and old ones How fast a galaxy spins helps define its shape. Also differ in size, mass, colour, brightness and speed of spin Differences are determined by the number of stars, type of stars and the amount and type of gas and dust Also contain two types of distinct formation of stars called star clusters - globular cluster has a spherical shape containing up to a million or more stars - open cluster has a few hundred to tens of thousands of stars